It has been recognized heretofor that molybdenum dioxide (MoO.sub.2) is a superior product to molybdenum trioxide (MoO.sub.3) in a number of ways. For instance, the molybdenum content of MoO.sub.2 (75%) is higher than that of MoO.sub.3 (66%). In addition, the specific gravity of MoO.sub.2 (6.44) is considerably higher than that of MoO.sub.3 (4.5). These factors favor MoO.sub.2 when packaging and shipping costs are considered. In addition, MoO.sub.2 is much less volatile than MoO.sub.3 when heated and this factor favors use of MoO.sub.2 as an alloying addition agent.
Conventional methods of roasting molybdenite produce molybdenum trioxide. Accordingly, the prior art sought to partially reduce MoO.sub.3 to MoO.sub.2 with reducing agents such as hydrogen using elevated temperatures. This partial reduction procedure involved a two-step method which resulted in increased costs and resort to an expensive agent, hydrogen, which is difficult to handle. Resort was also had to another two-step and tricky process involving the endothermic reaction of molybdenite (MoS.sub.2) with MoO.sub.3. This reaction requires at least six (6) gram-molecular weights of MoO.sub.3 to one of MoS.sub.2. Careful control of the trioxide to sulfide ratio is required to avoid incomplete roasting at too low a ratio and to avoid the formation of a low-melting sticky phase in the product at too high a ratio of MoO.sub.3 -to-MoS.sub.2. The presence in the concentrate of oil from the flotation process complicates control of the ratio. Such oil is frequently variable in amount and consumes oxygen at the roasting temperature which is usually about 700.degree. C..+-.50.degree. C. A process which could provide molybdenum dioxide directly from molybdenite in a single step would afford significant advantage as compared to the prior processes, particularly if the process were controllable. Such a controllable, single-step process is provided by the invention.